The technical field of this invention is corneal surgery and, in particular, the invention relates to systems and methods of repro filing the cornea of an eye by application of ablative radiation.
Recently, it has been demonstrated that changes in the refractive power of the eye can be achieved by laser ablation of the corneal surface. Such procedures, known as photorefractive keratectomy (PRK), involves the use of a nonthermal, high energy, laser radiation to sculpt the cornea into an ideal shape. For details, see, Marshall et al. "Photoablative Repro filing of the Cornea using an Excimer Laser: Photorefractive Keratectomy," Vol. 1, Lasers in Ophthalmology, pp. 21-48 (1986); and Tuff et al. "Stromal Remodeling Following Photorefractive Keratectomy," Vol. 1, Lasers in Ophthalmology, pp. 177-183 (1987), herein incorporated by reference.
The cornea of the eye comprises transparent avascular tissue. The cornea functions as both a protective, anterior membrane and a "window" through which light passes as it proceeds to the retina. The cornea is composed of a set of distinct layers: the outer epithelium, an anterior elastic lamina known as "Bowman's membrane," the cornea proper (or "stroma"), a posterior elastic lamina known as "Descemet's membrane", and the inner endothelium. The stroma is fibrous and constitutes the major portion of the cornea. Bowman's membrane, which forms the outer elastic lamina, is a rigid fibrillar structure not tending to cut or fracture, while Descemet's membrane, which forms the inner elastic lamina, is very brittle but elastic and has a tendency to curl. Together, the Bowman's and Descemet's membranes impart the necessary curvature to the stromal tissue. This curvature of the cornea constitutes an major component of the refractive power of the eye, thereby allowing objects to be imaged onto the retina.
The average adult cornea is about 0.65 mm thick at the periphery, and about 0.54 mm thick in the center. Photorefractive keratectomy involves sculpting the uppermost regions of the cornea, namely, the epithelium, Bowman's membrane, and the outer stroma. The epithelium consists of five or six layers of cells, and the underlying Bowman's membrane, is also a very thin structure. The corneal stroma accounts for about 90 percent of the corneal thickness. In performing PRK operations on the cornea, care must be taken to avoid damaging the underlying Descemet's membrane or endothelium.
In photorefractive keratectomies, a laser photoablation apparatus is used to change the curvature of the cornea, at least in the so-called "optical zone" or region of the cornea through which light must pass to enter the pupil and reach the retina. The size of the optical zone will, of course, vary from individual to individual, and will also vary based upon ambient light conditions (because the pupil will dilate and contract in response to ambient light). The extent of the sculpted region (and the depth of ablation) will depend on the amount of correction needed to achieve optimal vision. For example, correction of relatively mild myopia (nearsightedness) on the order of 2 Diopters requires only a modest flattening of the corneal curvature, which can be accomplished in a region of small cross-sectional area (e.g., affecting a circular region of the cornea in front of the pupil less than 5 millimeters in diameter). However, when more complicated refractive errors, such as more severe myopia, hyperopia (farsightedness) or astigmatisms, are corrected by PRK procedures, the sculpted area will extend across a much larger portion of the cornea, e.g., affecting a region as large as 8 mm in diameter or more.
One approach to performing PRK procedures is to employ an optical system which varies the size of the exposure area over time to effect a new profiling as the result of accumulated ablation steps. In one embodiment of such a "variable exposure area" system, a beam-shaping stop or window is moved axially along the beam to increase or decrease the region of cornea on which the laser radiation is incident. By progressively varying the size of the exposed region, a desired photoablation profile is established on the surface. For further details on this technique, see U.S. Pat. No. 4,941,093 issued to Marshall et al. on Jul. 10, 1990, herein incorporated by reference.
Another new technique for corneal reshaping involves the use of a beam-shaping mask which is disposed between the laser and the surface. In one embodiment, the mask provides a predefined profile of resistance to erosion by laser radiation whereby a portion of the laser radiation is selectively absorbed and another portion is transmitted to the surface in accordance with the mask profile. For further disclosures of such masking techniques, see U.S. Pat. No. 4,856,513 issued to Muller on Aug. 15, 1989; U.S. Pat. No. 4,994,058 issued to Raven et al. on Feb. 19, 1991; U.S. Pat. No. 5,019,074 issued to Muller on May 28, 1991, and U.S. Pat. No. 5,324,281, issued to Muller on Jun. 28, 1994, all of which are incorporated herein by reference.
To correct hyperopia, in particular, it is necessary to increase (steepen) the curvature of the cornea. This can be accomplished with either a movable stop system, using a stop which blocks the central portion of the beam, or with a beam-shaping mask which transmits an annulus of light. In the moving stop system, the inner diameter of the annulus typically is varied such that more light is transmitted to the periphery of the cornea. Similarly, with an erodible mask or the like, the mask operates to transmit an annulus of varying width over time with the same effect.
Hyperopia correction, which can require significant sculpting in a ring-like region having a diameter of about 4 mm to 8 mm, places additional demands on the engineering design of PRK apparatus, which normally must be met by increasing the size and/or power of the laser source. Increasing the power of the laser to compensate for inefficiencies in beam deliver is undesirable because large annular beams deliver a larger total amount of energy to the cornea per pulse.
Moreover, in performing hyperopia and/or astigmatic corrections, it is also often desirable to create a "blend zone" at the periphery of the sculpted region. Such blend zones provide an edge-smoothing effect where there would otherwise be a sharp circular (or elliptical) recess of a depth proportional to the magnitude of dioptric correction. Because sharp edges tend to induce regression of the reprofiled curvature, a blend zone provides an outer, profile-smoothing, region contiguous with the curvature-corrected zone. However, this blend zone further taxes the PRK apparatus because the zone must be created, at least in part, beyond the outer edge of the sculpted region and therefore can extend the overall diameter of the reprofiled region to as large as 10 millimeters. The need for a blend zone further fuels demand for laser beams having large and uniform cross-sectional fluences.
In addition, the formation of a blend zone at the outer periphery of the reprofiled cornea typically requires a reverse procedure, at least when a movable stop is employed, to perform hyperopia and/or astigmatic corrections. Either a different type of stop or an adjustable iris-type mechanism typically is used to deliver an annular pattern of radiation of progressively decreasing intensity to the outermost portions of the blend zone. This second stage in the procedure typically doubles the overall time that a patient must remain motionless with his or her eye aligned with the apparatus.
There exists a need for better PRK apparatus for refractive correction of myopia, hyperopia and/or astigmatism, as well as creation of blend zones, without resort to larger lasers. There also exists a need for systems that can achieve desired blend zones at the periphery of hyperopic or astigmatic correction regions with less waste of laser energy. In addition, a PRK system that could facilitate simplified and/or quicker formation of peripheral blend zones would satisfy a long-felt need in the art.